Many communication applications and hard disk drive units, particularly a read channel found in hard disk drives, require some sort of variable gain that is exponentially proportional to an input control voltage. Since on a dB scale, the gain curve becomes a straight line; this is commonly referred to as “linear-in-dB.” Examples of where a linear-in-dB variable gain is required are in transceivers for cellular phones as well as in read channels. Most VGA circuits accomplish the desired linear-in-dB behavior by, one way or another, exploiting the exponential characteristic of a bipolar transistor. A well-known technique relies on the fact that the ratio of the collector currents of a bipolar differential pair is exponentially dependent on the differential input voltage. FIG. 1 shows a differential pair of bipolar transistors Q1 and Q2 with common emitters biased by a tail current “Itail” from current source 5 and their bases controlled by a differential input voltage Vin+, Vin− at differential inputs 1 and 2. The relationship between the collector currents Iout+ and Iout− at the outputs 3 and 4 and the input voltages Vin+ and Vin− can be described as                                           I                          out              -                                            I                          out              +                                      =                  e                                    q              ⁡                              (                                                      V                                                                  i                        ⁢                                                                                                   ⁢                        n                                            +                                                        -                                      V                                                                  i                        ⁢                                                                                                   ⁢                        n                                            -                                                                      )                                      kT                                              (        1        )            where q is the charge of the electron, k is Boltzmann's constant, and T represents the absolute temperature. The fact that it is the ratio of the collector currents that exhibits the linear-in-dB behavior enables the bipolar differential pair to be used for a wide range of variable gain circuits that rely on current ratios to set their gain.
Some VGA circuits employ a comparison circuit that compares a common mode voltage Vcm to a predetermined voltage, and if there is a difference between the common mode voltage Vcm and the predetermined voltage, a feedback signal is employed to either raise or lower the common mode voltage Vcm so that Vcm approximately equals the predetermined voltage. This technique introduces problems in that instability can occur.